Bluff body noise control

ABSTRACT

The present invention provides a method of and apparatus for, reducing noise caused by the interaction of airflow between two bluff bodies which are in a generally tandem arrangement, the method comprising providing flow control such that the peak turbulence is at least partially displaced from, or reduced at, the surface of the downstream body. The invention also provides an aircraft noise reduction device, and a method of using such a device, comprising a flow control apparatus ( 2 ) arranged to be positioned downstream of a flow-facing element ( 1 ), wherein the flow control apparatus is arranged, in use, to reduce noise induced by unsteady flow downstream of the flow-facing element, and wherein the flow control apparatus ( 2 ) is arranged to be moveable in relation to the flow-facing element ( 1 ) between a stowed position and a deployed position.

BACKGROUND OF THE INVENTION

The present invention relates to noise-reduction apparatus for use on anaircraft.

More particularly, but not exclusively, the invention also relates to amethod of, and apparatus for, reducing noise generated by theinteraction of the landing gear or parts thereof and the air flowingpast it during flight, take-off and/or landing.

The invention also relates to an aircraft noise reduction devicecomprising a flow control apparatus arranged to be positioned downstreamof a flow-facing element, possibly on a landing gear and a method ofreducing noise caused by landing gear on an aircraft.

Flow around bodies generates noise, which is detrimental in particularaerodynamic applications, for example where low noise emissions are adesign requirement. One such application where the level of noiseemissions is important is in the design of commercial aircraft. Over thepast decades engine noise has been significantly reduced, for example bythe introduction of high-bypass ratio turbofan engines. However,maintaining the minimum engine ground clearance with such high-bypassratio turbo fan engines results in longer landing gear.

Thus, landing gear on commercial aircraft have been identified as majornoise contributors during approach and landing. The design of a landinggear is primarily based on its structural and dynamic function. Thesestringent requirements make it extremely difficult to have a totallyaerodynamic gear. This complex geometric design gives rise to unsteadyflow which leads to unwanted noise generation.

WO2009/053745 A1 discloses the use of a splitter plate to reduce noiseinduced by unsteady flow downstream of a flow-facing element. Thisdocument also discloses preliminary experiments which were carried outto evaluate the effectiveness of using a splitter plate.

Fairings have been proposed as a means of reducing landing gear noise.For example, a noise reduction fairing for an aircraft landing gear isdisclosed in WO 01/04003A1. Such noise reduction fairings at leastpartially shield downstream components such as strut stays and actuatorsfrom high-speed flow.

The present invention seeks to mitigate the above-mentioned problemsand/or to provide an improved noise-reduction method or apparatus.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides a method ofreducing noise caused by the interaction of airflow between two bluffbodies which are in a generally tandem arrangement, the methodcomprising providing flow control such that the peak turbulence is atleast partially displaced from, or reduced at, the surface of thedownstream body. In this context a tandem arrangement is an arrangementof two bluff bodies in series in the streamwise direction. The two bluffbodies do not have to be directly upstream/downstream of each other(i.e. aligned with each other in the streamwise direction) but can be atan oblique angle to each other (i.e. not aligned with each other in thestreamwise direction).

Peak turbulence can be examined by looking at the Reynolds stresses orperturbed velocities. In the present invention, the use of flow controlcauses the position where peak turbulence occurs (seen as peak perturbedvelocities and peak Reynolds stresses) to move away from the centerlineof the downstream body such that the peak turbulence is displaced fromthe surface of the downstream body.

The applicants have recognised that when an unsteady wake from anupstream body impinges on a downstream body, the resultant interactionnoise can be significant.

The skilled person will appreciate that a bluff body may be generallycharacterised as any body where there is significant flow separation anda generally unsteady wake.

The flow control may be arranged to break down large flow structures inthe flow between, and/or around, the bluff bodies.

The flow control may be provided by a flow control device on either theupstream or downstream body.

The flow control may comprise the use of blowing. For example, the flowcontrol device may be arranged to provide distributed blowing throughthe surface of one of the bodies.

The flow control device may form part of the bluff body. In other words,the flow control device may be contained within the bluff body.

The flow control device may comprise a series of plenum chambersdistributed around one of the bluff bodies. For example, plenum chambersmay be distributed at a variety of angular positions around the body.This gives a more uniform distribution of blowing.

Blowing may be applied to both sides of the body from 60 degrees to 150degrees as measured from the leading-edge of the body.

The positions where blowing is applied can be optimised depending on theairflow speed and the geometry configuration of the bluff bodies.

Each plenum chamber may, for example, be fed by a cross-drilled pipe.

The plenum chambers may be covered by air-permeable plate. This allowsair to pass through the plate. For example, a perforated steel plate ora sintered bronze plate.

The air-permeable plate may have a small pore size. This may enable alarge pressure loss coefficient and/or less variation in permeationvelocity across it and/or a higher pressure differential across theperforated plate for a given permeation velocity. A small pore size alsoproduces a lower volume of high frequency sound (hiss) from the airbeing blown through the pores.

The porosity of the plate may be approximately 30%. Porosity is definedas the ratio of open area of the plate to total area of the plate.

The pore size and porosity may be optimised for each airflow speed andgeometry configuration of the bluff bodies.

An embodiment of the first aspect of the invention concerns a method ofreducing noise caused by an airborne aircraft resulting from theinteraction of airflow between an upstream bluff body and a downstreambluff body positioned in a generally tandem arrangement in thestreamwise direction, wherein the method comprises the step of blowing aplurality of jets of air from at least one of the upstream bluff bodyand the downstream bluff body, the jets of air being blown fromdifferent positions distributed over the surface of the bluff body. Suchan arrangement of distributed blowing may act to provide air flowcontrol such that the peak turbulence downstream of the upstream body,caused by air flowing past the upstream body, is reduced and/ordisplaced to a position further from the downstream bluff body. Suchreduction or displacement in the peak turbulence downstream of theupstream bluff body acts to reduce noise, that would otherwise be causedby the interaction of the airflow between the upstream bluff body andthe downstream bluff body.

According to a second aspect of the invention there is also provided anoise reduction apparatus for use on a bluff body which, in use, isarranged in a generally tandem arrangement with a further bluff body,the apparatus comprising a flow control device arranged such that thepeak turbulence is at least partially displaced from, or reduced at, thesurface of the downstream body. In this context a tandem arrangement isan arrangement of two bluff bodies in series in the streamwisedirection. The two bluff bodies do not have to be directlyupstream/downstream of each other (i.e. aligned with each other in thestreamwise direction) but can be at an oblique angle to each other (i.e.not aligned with each other in the streamwise direction).

An embodiment of the second aspect of the invention concerns an aircraftincluding a first bluff body and a second bluff body positioned in agenerally tandem arrangement such that when the aircraft is airborne thefirst bluff body is upstream of the second bluff body, wherein theaircraft includes a flow control device on at least one of the firstbluff body and the second bluff body, the flow control device beingarranged to blow a plurality of jets of air from different positionsdistributed over the surface of the bluff body.

It will be appreciated that any features described with reference to themethod according to the first aspect of the invention are equallyapplicable to the noise reduction apparatus of the second aspect of theinvention.

According to a third aspect of the invention there is also provided anaircraft noise reduction device comprising a flow control apparatusarranged to be positioned downstream of a flow-facing element, whereinthe flow control apparatus is arranged, in use, to reduce noise inducedby unsteady flow downstream of the flow-facing element, and wherein theflow control apparatus is arranged to be moveable in relation to theflow-facing element between a stowed position and a deployed position.

The applicants have found that unsteady flow, around and in the wake ofa flow-facing element can cause a significant contribution to creationof broadband noise. In particular, it has been noted that unsteadyvelocity fluctuations and/or net lift forces generated in the flow maybe a key noise generating mechanism. As such, embodiments of theinvention utilise a flow control apparatus downstream of the flow-facingelement to reduce broadband noise. In particular, the flow controlapparatus may be arranged to reduce the flow fluctuations due torelatively large scale flow structures in the wake. For example, theflow control apparatus may be arranged to suppress vortex sheddingdownstream of the flow-facing element. Allowing the flow controlapparatus to be stowed while not in use is advantageous as, for example,when used on a landing gear, it allows deflection and articulation ofthe landing gear. It also allows stowage/retraction of the landing gearwhile the flow control apparatus is installed thereon. In other words,movement of the landing gear is not impeded by a flow control apparatus.It also means there is more space available in the very limited stowagespace of a landing gear bay.

The flow control apparatus may be a passive flow control apparatus. Apassive flow control apparatus may be optimised to provide the desiredflow control in a particular phase of flight. For example the flowcontrol apparatus may be optimised to provide the maximum noisereduction in flow conditions that would occur during approach andlanding.

The flow control apparatus may comprise a splitter plate arranged toextend downstream of the flow-facing element when in the deployedposition. Splitter plates (alternatively referred to as split plates)are a known means of aerodynamic flow control and have been primarilyused to modify the separated wake behind bluff bodies. Splitter platesgenerally extend from the centre-line of the downstream face of thebluff body.

When in the deployed position, the splitter plate may extend in asubstantially radial direction with respect to the flow-facing element.It may be substantially aligned with the free stream airflow.

When in the stowed position, the splitter plate may be orientated to atleast partly align with the flow-facing element. This allows theeffective space taken up by the flow control apparatus to be reduced.When on a landing gear, this allows the landing gear to be retractedmore easily.

The flow control apparatus may be made from a flexible material suchthat the flow control apparatus is deformable between the stowed anddeployed positions.

The flow control apparatus may be made from a resiliently flexiblematerial such that the flow control apparatus naturally assumes thedeployed position. This means that no active control is required todeploy the flow control apparatus.

The flow control apparatus may be made from a material with a sufficientstiffness to be maintained in the deployed position during use.

The flow control apparatus may be made from rubber.

The flow control apparatus may comprise brushes with resilientlydeformable bristles.

The bristles may be arranged to be mounted on a downstream side of theflow-facing element such that the bristles extend downstream of theflow-facing element when the flow control apparatus is in the deployedposition.

The flow control apparatus may be arranged to be automatically movedbetween the stowed and deployed positions when the flow-facing elementis moved from its stowed and deployed positions. This means that noactive control (other than control of the stowing/deployment of theflow-facing element) is required to cause stowing/deployment of the flowcontrol apparatus.

The flow-facing element may comprise an aircraft structural element. Forexample the flow-facing element may be a strut. Alternatively, theflow-facing element may comprise a fairing, for locating upstream of astructural element such that, in use, airflow is at least partiallydiverted away from the structural element, and the flow controlapparatus may be provided between the fairing and the structuralelement. Such an arrangement may help reduce self-noise which mayotherwise be produced by the fairing.

Where the flow-facing element is a fairing, the flow control apparatusmay be arranged to reduce re-circulating flow between the fairing andthe structural element. In some embodiments a splitter plate may bearranged such that it is also adapted to secure the fairing to thestructural member. The structural element may comprise a component of anaircraft landing gear.

The flow control apparatus may be arranged to be automatically movedbetween the stowed and deployed positions by a force applied by anelement of the landing gear when the landing gear is moved from itsstowed and deployed positions. This means that the flow controlapparatus can be urged into the stowed position by elements of thelanding gear moving into the landing gear stowed position.

The flow control apparatus may be arranged to suppress vortex sheddingdownstream of the flow-facing element.

The splitter plate may have a length which is less than, equal to orgreater than the streamwise length of the structural element.

The invention also provides an aircraft landing gear comprising a noisereduction apparatus or an aircraft noise reduction device as describedabove.

The invention also provides an aircraft comprising a noise reductionapparatus or an aircraft noise reduction device or an aircraft landinggear as described above.

According to a fourth aspect of the invention there is also provided amethod of reducing noise caused by landing gear on an aircraft includingthe steps of identifying a part of the landing gear that contributes tothe noise generated by the landing gear when in flight, and providing anoise reduction apparatus or an aircraft noise reduction deviceaccording as described above for reducing the noise generated by saidpart.

It will of course be appreciated that features described in relation toone aspect of the present invention may be incorporated into otheraspects of the present invention. For example, the method of theinvention may incorporate any of the features described with referenceto the apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying schematic drawings ofwhich:

FIG. 1 a shows, in plan view, a noise reduction apparatus in accordancewith an embodiment of the first aspect of the invention, which utilisesdistributed blowing on a cylinder upstream of an H-beam;

FIG. 1 b shows, in plan view, a noise reduction apparatus in accordancewith an embodiment of the first aspect of the invention, which utilisesdistributed blowing on a cylinder downstream of an H-beam;

FIG. 2 shows an anechoic chamber, used for measuring noise levels;

FIG. 3 shows a cut away section of a cylinder model, showing 3 plenumchambers;

FIG. 4 shows, schematically, a perspective view of a cylinder model,showing cross-drilled pipes for feeding air to the plenum chambers;

FIG. 5 is a graph showing mean velocity profiles in the wake at x=0.079m;

FIGS. 6 a to 6 f are graphical representations of the Reynolds stresses,comparing the use of blowing to no blowing;

FIGS. 7 a to 7 c are graphs showing the profiles of Reynolds stressesacross the wake;

FIG. 8 is a graph of sound pressure levels (SPL) against frequency fordifferent flow rates of blowing air for the cylinder upstream of theH-beam;

FIG. 9 is a graph of sound pressure levels (SPL) against frequency forthe cylinder upstream of the H-beam;

FIG. 10 a shows, in plan view, a noise reduction device in accordancewith an embodiment of the invention which utilises a stowable splitterplate, shown in its deployed position;

FIG. 10 b shows, in plan view, a noise reduction device of

FIG. 11 shows, in plan view, a noise reduction device in accordance withan embodiment of the invention which utilises a stowable splitter platemade from brushes, shown in its deployed position; and

FIG. 12 shows, in plan view, a noise reduction device in accordance withan embodiment of the invention which utilises a noise reduction fairingand stowable splitter plate, shown in its deployed position.

DETAILED DESCRIPTION

While previous work has focused on drag reduction or tonal noisereduction, this work investigates controlling broadband interactionnoise between bluff body components. An experimental investigation wasconducted to determine how the application of blowing to the cylindermodified the aerodynamics and acoustics of two bluff bodies in adirectly tandem arrangement in the streamwise direction.

The configurations investigation were an H-beam 6 and a cylinder 7 intandem, one with the cylinder 7 directly upstream (FIG. 1 a) and thesecond with the cylinder 7 directly downstream of the H-beam 6 (FIG. 1b).

Preliminary experiments were carried out to evaluate the effectivenessof embodiments of the invention. An example will now be described toillustrate the effectiveness of providing a flow control apparatusbetween the fairing and the structural element and to demonstrate thatembodiments of the invention are suitable for use as an aircraftnoise-reduction apparatus.

In the following description, the following nomenclature and symbolswill be used:

-   A Blowing area, m²-   Cμ Blowing coefficient={dot over (m)}V_(j)/qS (where V_(j) is the    velocity of the jet, in m/s, and q is the dynamic pressure, in    kg/ms²)-   {dot over (m)} Mass flow rate, kg/s-   {dot over (q)} Flow rate, m³/s-   S Planform area, m²-   Component separation distance, m-   u, v, w Cartesian components of velocity vector, m/s-   u′u′ Component of Reynolds stress tensor, m²/s²-   V Magnitude of velocity, m/s-   x, y, z Cartesian coordinates (x positive downstream, y positive to    port, z positive down)-   φ Cylinder diameter, m

The use of distributed blowing to reduce this source of noise wasinvestigated in a series of experiments. The two bluff bodies in tandemwere a cylinder 7 and an H-beam 6. Two configurations were tested, onewith the cylinder 7 directly upstream of the H-beam 6 and the other withthe H-beam 6 directly upstream of the cylinder 7. This modelled theinteraction noise due to large perturbations in the wake generated by anupstream component inducing unsteady pressure fluctuations on adownstream component. In both cases distributed blowing was applied tothe cylinder 7. The objective of the test was to reduce the interactionnoise of the two components by the use of blowing. Blowing was used tobreak down the large flow structures and to displace the peak turbulenceaway from the surface of the downstream component.

Microphone measurements were made in an anechoic chamber 8, shown inFIG. 2. The loudest configuration was the cylinder 7 upstream of theH-beam 6. Blowing produced a reduction of 16 dB at a frequency of 70 Hz.There was a broadband reduction of up to 4 dB up to a frequency of 10kHz. Only a low pressure of blowing air is needed. This is because, theair flowing over the cylinder 7 is less than atmospheric pressure soonly a low pressure jet is needed to expel air from the cylinder 7. Inaddition, only a relatively small blowing rate was required to providethe noise reduction. A blowing coefficient of 1.4×10⁻³ was adequate toachieve a noise reduction.

Apparatus and Procedure Wind Tunnel

Particle image velocimetry experiments were conducted in the Universityof Southampton's low speed 0.9 m×0.6 m wind tunnel. The wind tunnel hasa closed working section and is of an open circuit design. Endplateswere used to achieve a nominally two-dimensional flow around thecylinder 7 and H-beam 6. Free field acoustic measurements were conductedwith an open jet wind tunnel in an anechoic chamber 8 shown in FIG. 2.An arc 9 of 8 microphones was placed over the model.

Model Design and Test Configuration

A cylinder model 7 was designed to allow distributed blowing to beapplied through the surface of the model. The model was made from acarbon fibre cylinder 7 and contained 12 different plenum chambers 10.The diameter of the cylinder 7 was 0.1 m. The angular positions of theplenum chambers 10 were 60 to 90 deg., 90 to 120 deg. and 120 to 150deg. as shown in FIG. 3.

The plenum chambers 10 were further divided at the half-span of themodel. There were holes on the cylinder 7 surface to allow for theplacement of on-surface microphones.

The wind tunnel model is shown in FIG. 4. The design of a series ofplenum chambers 10 ensured a more uniform distribution of blowing.Blowing was applied to both sides of the cylinder 7 from 60 degrees to150 degrees as measured from the leading-edge of the cylinder 7. Eachplenum chamber 10 was fed by a cross-drilled pipe 11.

The plenum chambers 10 were covered by a permeable plate 12 whichallowed air to pass through it. Firstly, a perforated steel plate wastested. Secondly, a sintered bronze plate was tested. The sintered plateallowed a small pore size and therefore a large pressure losscoefficient, which resulted in less variation in permeation velocityacross it. It also resulted in a higher pressure differential across theperforated plate 12 for a given permeation velocity. The porosity ofboth plates 12 was 30%. The pore size for the perforated steel plate was1 mm and for the sintered bronze plate was 30 μm. It has been found thatusing a pore size of 12 μm produces a low volume high frequency (hiss)sound. Such “micro-perforated” plates were found to be beneficial inreducing the far-field radiating noise that was due to the blowingnoise.

The dimensions of the H-beam 6 were 0.1 m×0.1 m and it was extruded fromaluminium. The default separation distance between the two components(s) was 0.3 m (3φ). The Reynolds number of this investigation wasbetween 1.3×10⁵ and 2.6×10⁵.

The compressor used to supply the air was a double-flow side channelcompressor made by Rietschle. The maximum pressure difference was ±17000N/m² (170 mbar). The maximum mass flow rate was 0.078 kg/s. A soundabsorption settling tank was used for sound attenuation. The settlingtank consisted of a large box with two ducts offset from each other tominimise the direct transmission of sound from the compressor. Thechamber 8 was lined with an open cell porous material for soundabsorption.

Measurements

The system used for the particle image velocimetry (PIV) measurements isknown, for example as described in “Angland, D., Zhang, X., Chow, L. C.,and Molin, N., “Measurements of Flow around a Flap Side-Edge with PorousEdge Treatment,” AIAA Paper 2006-0213, 2006”.

The laser sheet was pointed in a horizontal plane through a glasswindow, which makes up one wall of the working section. The camera wasplaced above the transparent wind tunnel roof. An adaptive crosscorrelation was performed on interrogation areas measuring 16×16 pixels.The horizontal and vertical overlap was 75%. A peak validation of 1.2was used to reject spurious vectors. These time averaged data wereaveraged over 500 images sampled at 2 Hz. The image size was 110 mm×90mm with a spatial resolution of 0.35 mm in both directions.

The microphone measurements in the anechoic chamber 8 were conductedusing eight Behringer ECM8000 microphones mounted on an arc. Thefrequency response of the microphones was from 15 Hz to 20 kHz. Themicrophones were powered by a DIGIMAX FS preamplifier. These data weresampled at a frequency of 44.1 kHz with a block size of 8192 averagedover 100 blocks. The 8 microphones were spaced equally on the arc from90 deg. to 157 deg. measured from the freestream velocity vector.

Sample of Results Particle Image Velocimetry

Particle image velocimetry (PIV) was used to determine the time-averagedflowfield and the turbulence statistics. The time averaged flowfieldallowed the profiles in the wake to be determined and an estimation ofthe momentum deficit or addition due to the application of blowing. Forcomparison a solid wall condition with no blowing was used. Thisprovided a baseline condition with which to compare the cylinder 7 withblowing applied.

Mean velocity profiles in the wake at x=0.079 m are shown in FIG. 5. Theeffect of blowing was most influential on the low speed, low pressureseparated flow behind the cylinder 7. The blowing flow rate wasinsufficient to promote significant early separation of the flow fromthe surface of the cylinder 7. However, it did have an influence on thelow speed flow in the wake immediately aft of the cylinder 7. Thevelocity profiles showed a widening of the wake to the sides of thecylinder 7 with blowing applied. Another effect was an increase in thevelocity gradient across the shear layer. The use of blowing alsoproduced a decrease in the momentum deficit immediately aft of thecylinder 7. This was an effect of blowing normal to the cylinder 7surface. However, due to the spreading of the wake the overall momentumdeficit increased.

Examining the Reynolds stresses helped determine how the application ofblowing changed the structures in the wake and altered the shear layer.In two dimensional PIV three components of the Reynolds stress tensorcan be determined, i.e. u′u′, u′w′ and w′w′. u′ and w′ are the perturbedvelocities about the mean flow. Therefore they are a measure of themagnitude of unsteady velocity fluctuations in the wake. The unsteadywake impinging on the downstream component is a source of significantadditional noise. To reduce the interaction noise the largeperturbations in the wake need to be reduced. These are shown in FIG. 6(a), (c) and (e) for the solid wall configuration. The shear layers atthe edge of the wake behind the cylinder 7 showed significant spreadingaft of the cylinder 7.

The Reynolds stresses in the wake with blowing applied is shown in FIG.6( b), (d) and (f). With blowing, the magnitude of the components of thestress tensor had reduced and their spatial extent had also beenreduced. The profiles of the Reynolds stresses across the wake are shownin FIG. 7. The effect of applying blowing through the perforatedmaterial was to reduce the magnitude of Reynolds stress in the wake andshear layer and to displace the peak of maximum stress further away fromthe centerline.

The effect of blowing on the upstream cylinder 7 was to breakdown thelarge flow structures in the wake which was evidenced by the reductionin the velocity fluctuations in the shear layer and wake. This reductionin wake strength reduced the interaction noise when the turbulent wakeimpinged on the downstream component. To determine the acousticreduction, free-field microphone measurements were made in an anechoicchamber 8.

Acoustic Measurements

Acoustic measurements were made in an anechoic chamber 8 with an openjet wind tunnel. The changes in sound pressure levels were averaged overthe four microphones above the model in the overhead position. Thesemicrophones recorded the loudest model noise. No acoustic data could beobtained in the rearward arc due to the wake of the model impinging onthe microphones. The first configuration tested was the cylinder 7placed upstream of the H-beam 6. The blowing was applied to the cylinder7. A plot of sound pressure level (SPL) versus frequency is shown inFIG. 8 with different flow rates.

The maximum reduction was 16 dB centered around 70 Hz which correspondedto a Strouhal number based on cylinder 7 diameter of 0.18 m. There wasalso a further broadband reduction from 800 Hz to 10 kHz of up to 4 dB.The largest flow rate tested produced additional blowing noise at 12 kHzthat was louder than the baseline configuration. From the aerodynamicflowfield investigation it was shown that blowing on the cylinder 7reduced the large velocity fluctuations in the wake due to breaking downthe large flow structures in the wake. This modified the wake thatimpinged on the H-beam 6 downstream resulted in significantly less noisebeen generated.

The second configuration tested was the H-beam 6 upstream of thecylinder 7. The blowing was applied to the cylinder 7 to displace theturbulent wake away from the downstream component. A plot of SPL versusfrequency is shown in FIG. 9. The peak reduction was 20 dB at afrequency of 300 Hz which corresponded to a Strouhal number of 0.75based on the height of the H-beam 6. The flow rate was 0.7×10⁻³ m³/s.There were broadband reductions from 20 Hz up to 3 kHz.

Above this frequency the blowing noise was louder than the noisegenerated by the wind tunnel model. This configuration of H-beam 6 andcylinder 7 produced very little noise at and above this frequency andall the energy was contained in the lower frequencies. However, it iswell known that high frequency noise dissipates much quicker than lowerfrequency noise. Hence, when distributed blowing is used on landing gearcomponents, the increase in high frequency noise would not be perceivedon the ground, depending on the pore size. This is because theadditional high frequency blowing noise is proportional to pore size.

CONCLUSION

Although, in the above example, the blowing is achieved by the use ofcross-drilled pipes 11 inside the cylinder 7 feeding air to plenumchambers 10, the blowing mechanism may, alternatively, simply comprisepipes attached to the element, each pipe having a series of nozzles inform of simple holes or slots. The holes or slots are distributed alonglength of the pipe (and therefore, along the length of the element) toform an array. Pressurised air is provided to the pipe to provideblowing from the nozzles in the form of a relatively small jetdownstream. As another alternative, the pipes may be embedded in thecylinder 7 structure.

The use of blowing to reduce interaction noise between two bluff bodycomponents was investigated. The tandem configuration producedsignificantly more noise compared to the isolated components. Theapplication of blowing to the cylinder 7 produced a noise reduction forboth configurations. The noisiest configuration was the cylinder 7placed upstream of the H-beam 6. Blowing applied to the cylinder 7reduced the large velocity fluctuations in the wake of the cylinder 7thereby reducing the interaction noise. The largest reduction was 16 dBat a freestream velocity of 40 m/s. The largest flow rate producedadditional high frequency noise at 12 kHz. Blowing on the cylinder 7when it was downstream of the H-beam 6 also produced a broadbandreduction in the interaction noise of up to 20 dB. The frequency rangeover which the blowing was successful in reducing the noise was from 20Hz to 3 kHz.

Practical Application on an Aircraft

When used on a landing gear of an aircraft, it is envisaged that theblowing air will be taken from a bleed air system of an aircraft.Alternatively, the air can be taken from a fan output or from an airscoop. The air will be supplied to plenum chambers distributed about anelement of the landing gear acting as a bluff body.

FIG. 10 a shows a noise reduction device in accordance with anembodiment of the third aspect of the invention, in its deployedposition.

The noise reduction device comprises a structural element 1 which isexposed, in use, to an airflow V. In other words, the structural element1 is flow-facing. V. may be assumed to be the free stream airflow. Inthe case where the structural element 1 is a landing gear component itwill be appreciated that it may be deployable, such that it is only beexposed to the airflow V_(∞) during take-off, landing and approach.

The structural element 1 is a bluff body, in this case a H-beam 6. Thestructural element has a length in the streamwise direction of W. Theskilled person will appreciate that a bluff body may be generallycharacterised as any body where there is significant flow separation anda generally unsteady wake.

The noise reduction device further comprises a flow control apparatus inthe form of a splitter plate 2 of streamwise length L. The splitterplate 2 is a resiliently flexible plate attached to the downstream sideof the structural element 1. When in the deployed position, the splitterplate 2 extends perpendicularly from the downstream surface of thestructural element 1 and is located on the centre line of the element.

The splitter plate 2 is made of a soft rubber material.

The splitter plate 2 is arranged such that when it is in the deployedposition, it is substantially aligned with the free stream flow V. Whenmounted on an aircraft it may be convenient to simply align the splitterplate 2 with the longitudinal axis of the aircraft, since this is areasonable approximation to the free stream airflow during approach andlanding.

The splitter plate 2 can be deformed into a stowed position (shown inFIG. 10 b) by a force exerted on it. In the case where the structuralelement 1 is a landing gear component it will be appreciated that theflow control apparatus may be deformed into its stowed position byforces exerted on it by elements of the landing gear (not shown) as theyare moved into their stowed position.

As is shown in FIG. 10 b, the flow control apparatus 2, when in thestowed position, is bent back towards the structural element 1 to acertain extent so that it takes up less effective space than when in thedeployed position. In other words, the flow control apparatus 2 extendsoutwards from the structural element a length less than its actuallength L.

As an alternative, as shown in FIG. 11, the splitter plate 2 maycomprise brushes with resiliently deformable bristles 5. The bristles 5are attached to the downstream side of the structural element 1. When inthe deployed position, the bristles 5 extend perpendicularly from thedownstream surface of the structural element 1 and are located on thecentre line of the element.

Preliminary experiments were carried out to evaluate the effectivenessof using a splitter plate. An H-beam 6 was tested as it is considered agood example of a simple bluff-body which produces noise over a broadrange of frequency spectrum. A splitter plate 2 having a length L,measured in the streamwise direction, was attached to the rear of theelement 1 having a length W. A selection of different splitter platelengths (L/W=1, L/W=2 and L/W=3), and a body without a splitter plate,were tested.

A comparison of flow visualisations with and without the presence of thesplitter plate showed that the presence of the splitter plate blockedinteraction between shear layers in the vicinity of the body. The shearlayers continued to converge downstream leading to a longer and widerwake. The splitter plate 2 delays the roll-up of vortices behind theelement 1 and interrupts the interaction of shear layers.

The Coefficient of Drag for each arrangement was also compared. Theaddition of the L/W=1 splitter plate resulted in a drop in thecoefficient of drag of C_(d)=0.47. Increasing the length of the splitterplate reduced the drag further by C_(d)=0.23 between L/W=1 and L/W=3.

Standard deviations of velocity plots were used to compare the unsteadyflow. The unsteadiness was concentrated around the H-beam 6 with thehighest velocity fluctuation just aft of it. In the L/W=1 configurationthe unsteadiness moved further downstream and away from the model.

The narrowband spectra were measured in an anechoic chamber 8 and plotscompared for the different configurations to show how the noisesignature of the model was affected. The L/W=0 case showed a strongtonal peak at a Strouhal Number (based upon the width of the body) of0.125 and broadband noise “hump” centered about a Strouhal Number of0.75. In the cases of L/W=1, L/W=2 and L/W=3 the tonal peak wassuppressed and the noise was reduced over the whole frequency range. Thesplitter plate configurations showed very similar noise spectra up to aStrouhal Number of 17.5. Above that frequency the L/W=2 configurationshowed marginally lower noise levels.

Source localization plots were used to identify where origin of thenoise reduction. The comparison between the plots showed that the H-beam6 is no longer the main noise source when the splitter plate is used.Rather, the noise source is located towards the trailing edge of thesplitter plate.

FIG. 12 shows a further embodiment of the invention in which theflow-facing component is a fairing 3, positioned upstream of astructural element 1 and arranged to at least partially divert the freestream airflow away from the element 1. Such fairings have been proposedfor noise reduction purposes. However, the applicants have recognisedthat in some circumstances the noise-reduction fairing 3 may itselfcontribute to the total broadband noise of the aircraft. Thus, accordingto embodiments of the invention a splitter plate 2 is provided in acavity 4 defined between the fairing 3 and the element 1. The splitterplate 2 may conveniently be arranged to support the fairing 3 from thestructural element 1.

The splitter plate 2 reduces or eliminates vortex shedding from thefairing 3 and in turn reduces noise. As with the previous embodimentsthis is due to the splitter plate 2 blocking the interaction betweenopposing shear layers. The splitter plate 2 also reduced the interactionbetween the shear layers and the downstream element 1.

It will be appreciated that the flexible splitter plate 2 may beattached to the downstream side of the fairing 3 or the upstream side ofthe element 1 or to both. When the flow control apparatus is moved tothe stowed position (not shown), the apparatus 2 can either be urgedtoward the fairing 3, the element 1 or both.

Preliminary experiments were carried out to evaluate the effectivenessof using a splitter plate in the cavity between a fairing and astructural element. Three different sizes of elements 1 were used toinvestigate the possibility of reducing the size of the fairing 3 withrespect to the element 1. Aerodynamic and acoustic results wereperformed in wind tunnel and anechoic facilities.

In the configurations without the splitter plate 2 a re-circulatingregion of flow was observed in the cavity 4 between the fairing 3 andthe element 1 as the shear layer aft of the fairings' trailing edgeimpinged on the element part, rolling up inside the cavity 4. Theelement 1 was subjected to relatively high-speed flow due to the shearlayer interaction.

The application of the splitter plate for the two smaller elements 1blocked the interaction between the opposing shear layers and inhibitedthe shear layer from interacting with the element. As a result there-circulating flow inside the cavity 4 was reduced considerably. Thelarger element 1 was large enough for the shear layer to impinge on it,nevertheless the splitter plate 2 impeded the strong re-circulation flowwithin the cavity 4. Instead a low velocity wake was observed aft of theelement 1. The effect of this change in flow structure had an impact onthe noise produced. The source strength around the apparatus wassignificantly reduced as the magnitude of the velocities and theunsteadiness around the fairing 3 and the element 1 were lower, hencereducing the dipole strength attributed with the fluctuating lift forceson the apparatus. The strong shedding produced a strong tonal peak inthe noise measurements, increasing the overall noise signature. Thesplitter plate reduced or totally eliminated this tone. Theconfigurations involving the two smaller elements 1 reduced this tonalpeak by about 14 dB, measured from the ⅓-octave band spectra. The largerelement eliminated the tonal peak completely although a second smallertonal peak was observed at a high frequency.

Practical Application on an Aircraft

When used on a landing gear of an aircraft, it is envisaged that thesplitter plate would be attached to the downstream side of an element ofthe landing gear acting as a bluff body. When the landing gear isretracted into its stowed position, elements of the landing gear move inrelation to each other. The splitter plate will be moved/deflected byone or more of the landing gear elements if the splitter plate is in thepath of the moving landing gear elements. The force from the elementsacting on the splitter plate will cause the splitter plate to deflectand allow the landing gear to retract into its stowed position. Upondeployment of the landing gear, the landing gear elements will move andany elements causing the splitter plate to deflect will be moved out ofthe way allowing the splitter plate to adopt its deployed position.

Whilst the present invention has been described and illustrated withreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not specifically illustrated herein. It will beappreciated that various changes or modifications may be made withoutdeparting from the scope of the invention.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims. Moreover, it is to be understood thatsuch optional integers or features, whilst of possible benefit in someembodiments of the invention, may not be desirable, and may therefore beabsent, in other embodiments.

1. A method of reducing noise caused by the interaction of airflowbetween two bluff bodies which are in a generally tandem arrangement,the method comprising providing flow control such that the peakturbulence is at least partially displaced from, or reduced at, thesurface of the downstream body.
 2. A method of reducing noise as claimedin claim 1, wherein the flow control is provided by a flow controldevice on the upstream or downstream body.
 3. A method of reducing noiseas claimed in claim 1, wherein the flow control comprises the use ofblowing.
 4. A method of reducing noise as claimed in claim 3, whereinthe flow control device is arranged to provide distributed blowingthrough the surface of one of the bodies.
 5. A method of reducing noiseas claimed in claim 3, wherein the blowing is applied to both sides ofthe body from 60 degrees to 150 degrees as measured from theleading-edge of the body.
 6. A method of reducing noise as claimed inclaim 4, wherein the flow control device comprises a series of plenumchambers distributed around one of the bluff bodies.
 7. A method ofreducing noise as claimed in claim 6, wherein the plenum chambers arecovered by an air-permeable plate.
 8. A noise reduction apparatus foruse on a bluff body which, in use, is arranged in a generally tandemarrangement with a further bluff body, the apparatus comprising a flowcontrol device arranged such that the peak turbulence is at leastpartially displaced from, or reduced at, the surface of the downstreambody.
 9. An aircraft noise reduction device comprising a flow controlapparatus arranged to be positioned downstream of a flow-facing element,wherein the flow control apparatus is arranged, in use, to reduce noiseinduced by unsteady flow downstream of the flow-facing element, andwherein the flow control apparatus is arranged to be moveable inrelation to the flow-facing element between a stowed position and adeployed position.
 10. An aircraft noise reduction device as claimed inclaim 9, wherein the flow control apparatus comprises a splitter platearranged to extend downstream of the flow-facing element when in thedeployed position.
 11. An aircraft noise reduction device as claimed inclaim 10, wherein, when in the deployed position, the splitter plate issubstantially aligned with the free stream airflow.
 12. An aircraftnoise reduction device as claimed in claim 10, wherein, when in thestowed position, the splitter plate is orientated to at least partlyalign with the flow-facing element.
 13. An aircraft noise reductiondevice as claimed in claim 9, wherein the flow control apparatus is madefrom a flexible material such that the flow control apparatus isdeformable between the stowed and deployed positions.
 14. An aircraftnoise reduction device as claimed in claim 13, wherein the flow controlapparatus is made from a resiliently flexible material such that theflow control apparatus naturally assumes the deployed position.
 15. Anaircraft noise reduction device as claimed in claim 9, wherein the flowcontrol apparatus is made from rubber.
 16. An aircraft noise reductiondevice as claimed in claim 9, wherein the flow control apparatuscomprises brushes with resiliently deformable bristles.
 17. An aircraftnoise reduction device as claimed in claim 9, wherein the flow controlapparatus is arranged to be automatically moved between the stowed anddeployed positions when the flow-facing element is moved from its stowedand deployed positions.
 18. An aircraft noise reduction device asclaimed in claim 9, wherein the flow-facing element comprises anaircraft structural element.
 19. An aircraft noise reduction device asclaimed in claim 18 wherein the structural element comprises a componentof an aircraft landing gear.
 20. An aircraft noise reduction device asclaimed in claim 19, wherein the flow control apparatus is arranged tobe automatically moved between the stowed and deployed positions by aforce applied by an element of the landing gear when the landing gear ismoved from its stowed and deployed positions.
 21. An aircraft landinggear comprising a noise reduction apparatus as claimed in claim 8 or anaircraft noise reduction device.
 22. An aircraft comprising a noisereduction apparatus as claimed in claim
 8. 23. A method of reducingnoise caused by landing gear on an aircraft including the steps ofidentifying a part of the landing gear that contributes to the noisegenerated by the landing gear when in flight, and providing a noisereduction apparatus as claimed in claim 8.